机械英文论文热压成形技术对汽车高强度钢性能影响中英文对照.docx
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机械英文论文热压成形技术对汽车高强度钢性能影响中英文对照.docx
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机械英文论文热压成形技术对汽车高强度钢性能影响中英文对照
InfluenceofHotPressFormingTechniquesonPropertiesofVehicleHighStrengthSteels
CHANGYing,MENGZhaohuan,YINGLiang,LIXiaodong,MANing,HUPing
(SchoolofAutomotiveEngineering,StateKeyLaboratoryofStructuralAnalysisforIndustrialEquipment,
DalianUniversityofTechnology,Dalian116024,Liaoning,China)
Abstract:
Basedonthecombinationofmaterialsscienceandmechanicalengineering,hotpressformingprocessofthevehiclehighstrengthsteelswasanalyzed.Thehotformingprocessinclud-ed:
heatingalloysrapidlytoaustenitemicrostructures,stampingandcoolingtimely,maintainingpressureandquenching.Theresultsshowedthatmostofaustenitemicrostructurewaschangedintouniformmartensitebythehotpressformingwhilethesampleswereheatedat900。
Candquenched.Theoptimaltensilestrengthandyieldstrengthwereupto1530MPaand1000MPa,respectively,andtheshapedeformationreachedabout23%.Andspringbackdefectdidnothapp-eninthesamples.
Keywords:
highstrengthsteel;lightweight;hotforming;martensite
Asaneffectiveeconomicalenergymeasure,thelightweightdevelopmentdirectionofautomo-bilehasbecomeoneofthemostimportantresearchsubjectsintheautomotiveindustry.Therearethreemajorwaystoachieveautomobilelightweight:
optimizingvehicleframesandstruc-tures;makingvehiclebodyorframeofnewandalternativematerialstoreducethevehiclemass(Thehighandultrahighstrengthsteelcanbeusedasalternativematerialsbecauseofitsthinnerthickness);adoptingadvancedmanufacturingtechniquesforthesakeofautomobilelightwei-ght,suchasthickness-gradienthighstrengthsteel(HSS)ormetalbasedcompoundplatesbycon-tinuouspressingorhotpressforming[1].AlthoughHSShasbeenappliedinsomedomestictopgradevehicles,thekeyproducingtechnologieshavealwaysbeendominatedbyforeigncompan-ies,suchasAcelorCompany,soastoraisetheproductcostobviously.Bydomesticself-designedhotpressformingtechniquesandwater-coolingmould,theautomobileHSScanbeproducedtosubstituteforeignvehicleparts.
Ingeneral,withtheenhancementofsteelblank,smechanicalstrength,itsformabilityisworseneddramatically.ItisdifficulttoapplythetraditionalcoldstampingtechnologyintothefieldofpressingHSS.Thus,thehotstampingtechnologyofmartensiticsteelblankisappliedasanewtechnology,whichcombinesmetalthermoplasticformingmethodandwater-coolingmouldquenchingprinciple.Inthispaper,boronsteelblankwasformedandwater-coolingmouldwasquenchedsimultaneouslyduringtheprocessofhotstamping.Compared
withoriginalautomobilepearlitesteel[2],theautomobileHSSobtainedbyadvancedhotpressformingtechniquecanreduceabout30%ofthetotalvehiclemassandachievecomplexgeometries,highsecurityandmechanicalstrength.Thereasonisthataustenitemicrostructurewithoptimalplasticityandductilitycanbeobtainedbyhotpressformingathightemperature[3-5],andtheHSSwithbothexcellentmechanicalpropertiesandlightweightwillbeobtained
afterbeingformedandquenched[6-8].Theapplicationofhot-formedthinnerHSSplateswillbecomanimportantmeasuretorealizevehiclelightweight.
1ExperimentalSetup
InordertoformHSSathightemperature,andtoavoidcracksandspringback,thesam-plesneedrapidheatingandtransformcompletelyintostableaustenitemicrostructure.Andthen,samplesarepressedandcooledinself-madewater-coolingmould.FortheobtainedHS-Ssample,itsshape-freezingcharacterornospringbackdefectisanobviousadvantage,andmostofmicrostructureinthesampleismartensite.Thethicknessofsampleis1.6mm,andthemainelementsofHSSinthisexperimentareshowninTable1.
Table1Mainelementsofmaterialintheexperimen
22MnB5
C
Mn
Cr
Si
B
P
S
Al
Minimum
0.220
1.200
0.110
0.002
0.002
-
-
0.020
Maximum
0.250
1.400
0.200
0.005
0.005
0.020
0.005
0.050
Actualexperimentalprocedureincluded:
1)setdifferentheattreatmenttemperaturesintherangeof750to1000℃;2)putthesampleintotheheattreatedfurnacetobeheatedfor4minatacertaintemperature;3)removeitbymechanicalhandandputitintothehotformingmouldstobepressedquickly;4)simultaneously,itwaswater-cooledatabout30℃/sinthemound.Themechanicalpropertiesofsamplewereanalyzedbytensiletestsystemandthemicrostructureappearancewasanalyzedbymetallographicanalysisdevice.
TheshapeandsizeoftestsampleareshowninFig.1.
Fig1Theshapeandsizeofspecimen
2ResultsandDiscussion
MechanicalpropertiesofHSS(boronsteels)withdifferentthicknesses(1.0mm,1.6mm,2.0mm,2.5mm,3.0mmand4.0mm,respectively)werechecked(GBT16865-1997wasconsulted,andsampleswereselectedalong0℃,45℃and90℃rollingdirectionrespec-tively).Theunidirectionaltensiletests(basedonthemetaltensiletestingstandardofGBT228-2002)werefinished.ComparedwithUSIBOR1500,thevaluesofbasicmechanicalpropertiesforHSSwithdifferentthicknessesintheexperimentareshowninFig2.
Fig2showsthatafterwater-coolingquenching,thetensilestrengthandyieldstrengthofsamples(excepttheonewiththicknessof4.0mm)reached1500MPaand1000MPa,respectively.Thevaluesofthestrengthweretwicebetterthanthoseofsamplesbeforequenching,andnearlythesametothoseoftheplatesofthickness1.75mmfromAcelorCompany(USIBOR1500showninFig1).
Fig2Tensileandyieldstrengthofhighstrengthsteelswithdifferentthicknessesbeforeandafterquench–ing
Generally,hotpressformingofsamplesisoperatedabovetransitiontemperatureofmartensitemicrostructure.Theheatingtemperatureinthisexperimentwasintherangeof750to1000℃becauseittook3sorsoforthesamplestobedeliveredintheair.Andthen,basedonanalyzingtensilestrengthsRmofsamplesafterhot-formingatdifferenttemperatur-esandquenching,theoptimaltemperaturecanbefound.ItisshowninFig3.
Fig3Curveoftensilestrengthvspreheatingtemperature
FromFig3,itisobviousthatthevalueoftensilestrengthRmonlyreaches900MPaat750℃;theoptimalvalueis1530MPaat900℃,andthevaluewillfallastemperatureissetabove900℃.BasedonanalyzingmicrostructureandFe-Fe3Cphasediagram,sampleslayinthetransitionregionofferriteaustenitemicrostructurecoexistenceat750℃.Atthismoment,austenitehasappearedinmicrostructureofsamples,anditcanbetransformedintomartensitemicrostructurethroughwater-cooling.Sothemechanicalproperties,suchastensilestrengthandyieldstrength,willbeimproved.Thatistosay,tensilestrengthofsamplesisalittlehigherthanthatoforiginalones(Rmis600MPaorso).Thecontentofaustenitebecomeslargerastemperatureisraised,andthetensilestrengthwillbeimprovedgradually.Asfaras22MnB5steelisconcerned,theaustenitizingtemperatureisabout880℃.AsFig3shows,ifsamplesareheatedrapidlyto900℃andaircooledfor3,austenitemicrostructuresareobtainedcompletely.Thensamplesarehotformedandwater-coolingquenched,thefractionofmartensitemicrostructureinsamplesismorethan95%,sothecurveshowsapeak.However,astemperatureexceeds900℃,becausesuperheatdegreeistoolarge,micrograinsgrowsolargethatthetensilestrengthwilldecrease.Thushightem-peratureaustenitemicrostructure(obtainedassampleswereheatedrapidly)andgrainrefinementarethemainfactorstodeterminethemechanicalpropertiesofhighstrengthsteel-s.Inthispaper,differentfromthatinthelab,theinteractionmechanismsofmoldingandwater-coolingsystemonsamplesproducedintheproductionlinecanobjectivelyshowthemanufacturingpropertiesandmicrostructurecharacterofproductsinmass.
Asfarasthesamplesareconcerned,Aistheinitialanduntreatedsample;Bisthesamplewhichwasheatedat900℃for4min;Cisthesampleafterheattreatmentandwater-coolingquenching.ThedeformationofA,BandCare32%,24%and6%orso,respectively.Generally,Aiscomposedofmainpearliteandasmallamountofferrite,the
toughnessofwhichisbetterthanmartensite,soitsdeformationisrelativelybetter.Biscom-posedwiththehigh-temperaturetransitionalmicrostructureofaustenite,whosetoughnessisalsobetterthanmartensite,anddeformationislargerthanthelatter.Ciscomposedofover95%martensiteandlittleaustensite.Owingtoitshigherstrength,toughnessandplasticityofmartensitearelower,thatistosay,deformationofCisthelowestInFig4,whenthesamplewasheatedfor4minandstretchedat900℃,stress-straincurveandtestforcedisplacementcurvewereobtainedrespectively.
FromFig4(a),afterbeingheatedupto900℃,themicrostructureofsamplehasbeencompletelyturnedintoaustenite.Thevalueintheelasticdeformationstageofcurvewilltendtowardstheyieldpoint,followingtheaxialtestforcegraduallybeingincreased.Thatistosay,theobviousplasticdeformationofsamplewillbeginaftertheyieldpoint.Whenitiscontinuouslystretchedtillthepeakpointofcurve,theneckingofsamplewilloccur.Passingthepeak,thestress-strainrelationshipwillbecomemorecomplex.FromFig4(b),afterthecorrespondingpeak,thetestforcewillbereduced,alongwiththedecreasingcross-sectionalareaofsampletillthefracture.Itcanbeseenthattheappropriatetoughnessandplasticdeformationpropertiesofaustenitizingsampleat900℃willhelpHSSbehot-formedtocomplicatevehicleparts.Itisaneffectivem
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